1 . Field of the Invention
The invention generally relates to the analytical art and specifically to membrane-enclosed sensors for use in various methods of analysis.
2 . Description of the Prior Art
Membrane-enclosed sensors (also termed MES herein for short) are a known species of analytical devices frequently used for determining the concentration of an analytical species of interest in a fluid sample. One type of MES (a membrane-enclosed amperometric sensor or cell also termed "Clark Cell") is disclosed e.g. in U.S. Pat. No. 4,096,047, and the art mentioned therein, and is used in amperometric (also termed "polarographic" or "voltaic") analyses. Another type (a membrane-enclosed sensor sensitive to the flow rate of an analytical species through the membrane) is disclosed e.g. in U.S. Pat. No. 5,144,831 and EP-A-0429396 and the art discussed therein. Membrane separators for mass spectrophotometric, gas chromatographic and other techniques represent yet a further type of MES.
Generally, the membrane of an MES serves to separate a fluid substance (termed "external phase" herein) containing a species of analytical interest (also termed "analyte" herein) from a sensing means within the sensor. Generally, a gradient of concentration or partial pressure of the analyte, frequently but not necessarily a gaseous substance, such as molecular oxygen, hydrogen, ozone, carbon dioxide etc., is established across the membrane located between the fluid and the interior of the sensor. The flux of analyte diffusing across the membrane is then measured and related to the concentration or partial pressure of the analyte in the external phase or probe assuming that the membrane alone is responsible for limitation of the flux. The specific type of measuring means used depends upon the analytical method employed and is not a feature of the present invention.
The feature common to substantially all types of MES is their capacity to generate a signal related to the concentration of the analyte which frequently is a normally (i.e. under the pressure and temperature conditions of the measurement) gaseous substance of analytical interest in a fluid external phase which, in turn, may be liquid or gaseous.
The MES may, or may not, provide for selectively detecting or sensing more than one analyte (e.g. oxygen and hydrogen in mutual presence, cf. EP-A-0293541 or U.S. Pat. No. 4906339).
The membrane of a MES is frequently characterized as being "semipermeable" in the sense of permitting permeation of the analyte but substantially precluding permeation of a liquid or, sometimes, of another gaseous constituent of the external phase. Typical examples of such membranes are films or self-supporting strata made of organic polymers, e.g. of polytetrafluoroethylene, polyethylene, polyvinylidene chloride, or the like organic polymers, but membranes of other materials, such as foils of palladium for sensing of H.sub.2, may satisfy the requirements of a semipermeable membrane herein.
In any case, semipermeability of the membrane and even impermeability to liquids is not believed to be a critical aspect of the present invention because no liquid constituent might be present in the external sample and because the sensing method might be based upon differing diffusion rates of different gases. In such cases the function of the membrane is that of providing a diffusion impedance.
Generally, the operation of a MES is "destructive" (understood in the sense of "invasive" as opposed to "non-invasive") because the composition of the external phase will change in the close vicinity of the membrane due to "depletion", i.e. permeation or "extraction" of one or more analyte(s) from the external phase through the membrane into the sensing device where the analyte may be "consumed", i.e. converted into a different chemical species, vented, or removed from the system by another method. As a result, the external phase in close vicinity of the membrane becomes depleted of the analyte, and a certain "flow demand", or "minimum flow rate" discussed in more detail below, of the external phase will be required to achieve an acceptable fraction (say 99%) of the signal that would represent the theoretical or undepleted concentration of the analyte in the body of the external phase.
Typically, membranes of such types of MES are disk shaped films or foils sealed to the body of the sensor by a holding ring (cf. EP-A-0043611), and are supported from within the sensor but are necessarily unencumbered or only weakly supported on the external or sample side because of the need to allow unimpeded access of flowing external phase to the membrane.